1. Field of the Invention
The present invention relates to an image forming apparatus forming a toner image on a recording medium by using an electro-photographic system.
2. Description of the Related Art
Hitherto, there is known an image forming apparatus configured to transfer a toner image carried on a rotating endless belt-like intermediate transfer body (referred to as an ‘intermediate transfer belt’ hereinafter) to a recording medium at a transfer nip portion in which the intermediate transfer belt is in contact with a transfer rotating body (a transfer belt or a transfer roller). An intense electric field is generated in the transfer nip portion because high voltage is applied to the transfer rotating body in order to transfer the toner image from the intermediate transfer belt to the recording medium.
By the way, there is a case when the intermediate transfer belt vibrates during its rotation. If the intermediate transfer belt vibrates, a gap is possibly generated between a toner image carrying surface of the intermediate transfer belt and a recording medium at upstream (upstream in a recording medium conveying direction) of the transfer nip portion. Because the intense electric field is generated at the transfer nip portion as described above, an abnormal discharge tends to occur between the intermediate transfer belt and the recording medium if the gap is generated at upstream of the transfer nip portion. If the abnormal discharge occurs, the electrical charge of the toner carried on the intermediate transfer belt drops or is lost and the toner whose electrical charge has dropped or has been lost is hardly transferred from the intermediate transfer belt to the recording medium. Then, a defective image in which an image of a spot where the abnormal discharge has occurred is omitted (called as a void or the like) tends to be generated.
Then, Japanese Patent Application Laid-open No. 2002-82543 discloses a device that suppresses the vibration of the intermediate transfer belt that otherwise causes the abnormal discharge by disposing a belt guide member (press member) on an inner circumferential surface (back surface side opposite to the toner image carrying surface) of the intermediate transfer belt to press the intermediate transfer belt from the back surface thereof and to bulge out the belt to the side of the transfer rotating body. This arrangement makes it possible to deliver the recording medium to the transfer nip portion in a state in which the recording medium is adhered to the toner image carrying surface of the intermediate transfer belt without gap at upstream of the transfer nip portion.
However, even if the press member pressing the intermediate transfer belt is provided, there is a case when the recording medium conveyed to the transfer nip portion pushes up the intermediate transfer belt to a side opposite to the bulge-out side by an end portion thereof upstream in the conveying direction. In such a case, the intermediate transfer belt deflects and a gap is generated between the intermediate transfer belt and the end portion upstream in the conveying direction of the recording medium, possibly causing a defective image due to the abnormal discharge as described above.